1. Field of the Invention
The invention relates to information processing system organization. More particularly, the invention relates to a file management system. In still greater particularity, the invention relates to a labeling system. By way of further characterization, but not by way of limitation thereto, the invention is a method for storing multiple logical volumes of data on a single physical volume which method includes storing information on a writable label associated with the physical volume.
2. Description of the Related Art
The data processing industry stores large amounts of digital data on magnetic tapes, magnetic disks, optical tapes, optical disks, semiconductor memories, and the like. These media devices, generally referred to herein as physical volumes, may be implemented for example in the form of cassettes, cartridges, reels, modules and the like. The 3480 tape cartridge (developed by IBM Corp., Armonk N.Y.) is an example of an industry standard for magnetic tape storage media. The 3480 cartridge is a single--reel cartridge which houses a length of one--half inch wide magnetic tape.
In a typical data processing application, data is manipulated in the form of "data sets". Each data set comprises a collection of logically related information that is often termed a "logical volume". Typically, only one logical volume has been stored on a physical volume. Recently, the capacity of physical volumes has increased much faster than the size of a typical data set. For example, because of advances in media and recording technologies, the storage capacity of the 3480 form factor magnetic tape cartridge has increased by a factor of 25 since it was first introduced. This mismatch between logical volume size and physical volume capacity has prevented anticipated storage system performance improvements from being fully realized.
Potential performance improvements can be achieved in two key aspects of storage system operation. These include access time and cost of media. When a single logical volume is stored on a single physical volume, the time to access each logical volume includes the time required to exchange each physical volume between the read/write device and the storage location of the physical volume. The cost of the media used to store a single logical volume is equal to the cost of the physical volume, regardless of whether it utilizes the full capacity or only a small portion of it. If multiple logical volumes can be stored on a single physical volume, both the access time and the cost of storage can be reduced. Such a process is called "volume stacking" and the resulting physical volume is termed a "stacked volume". These logical volumes are also known as "virtual volumes" because they no longer must be associated with a single physical volume, but can be managed independently. The access time to multiple logical volumes on the stacked volume will be reduced by elimination of the time involved in the physical volume exchange. Media cost will be reduced by virtue of the efficient utilization of the full capacity of physical volumes, thereby reducing the total number of physical volumes required for storing a given number of data sets.
Unfortunately, the file management systems which manipulate the storage and retrieval of data sets were not originally designed to handle more than one logical volume per physical volume. Therefore, the potential performance advantages can be realized only if methods for managing these "virtual volumes" on stacked volume cartridges are devised. These methods must facilitate the tasks of generation, management, and distribution of information concerning relationships between logical volumes and physical volumes.
Prior art systems use a label adhered directly to the physical volume (e.g. cartridge) for identification. Serial or other identification numbers on the label can be used to distinguish the physical volumes from each other. A volume serial number (VOLSER) expressed in alpha--numeric characters and/or bar codes is traditionally used. In the traditional case of one logical volume stored on one physical volume, the VOLSER has existed in a one-to-one relationship with the data set name. Because the data set(s) stored in each physical volume is changeable, it is desirable to keep a record of its contents. Such records are frequently maintained in a computer system. Whenever specific information about the physical volume or the data stored therein is required, manual or automated apparatus read the identification label and transmit this information to the computer system. The computer system then matches a stored data record with the physical volume's VOLSER. In some systems, a summary of the contents of a cartridge is written on the media in the cartridge. In order to obtain this information, the cartridge must be accessed, loaded and read. An example utilizing such prior art labeling is StorageTek Model 4410 Automated Cartridge System (ACS) available from Storage Technology Corp., Louisville, Colo. The ACS includes automated apparatus to read identification labels on tape cartridges. The identification labels include a bar coded identification number. A look up table known as a Control Data Set (CDS) in a host computer provides a cross reference between the identification number and the contents of the tape cartridge.
While prior art systems, such as those employing the CDS referred to above could be utilized to manage stacked volume information, such prior art systems present disadvantages. A CDS can work effectively in a stacked volume situation but only if the system remains completely "closed." That is, as long as no additional physical volumes are introduced into, or removed from, the system, a CDS could be used. However, if physical volumes are entered or removed, major system performance degradation will be encountered. The performance degradation results from the time required to take the physical volume which is introduced into or removed from the system, mount it in a drive, and read and interpret its contents in order to update the CDS. Even if a range of serial numbers in a VOLSER set were dedicated to identify a stacked volume, the physical volume's contents would still have to be determined through the access, mounting, and reading processes. Corruption of the CDS is also possible which would require the reading of all tapes in the library to reconstruct the CDS. It would thus be desirable to have a method for tracking multiple data sets which, through volume stacking, are contained within a single physical volume.